This invention relates to the recovery of uranium from phosphate compounds and, more specifically, to the recovery of uranium from phosphoric acid produced by the acidulation of phosphate rock.
Most of the world's production of phosphate comes from marine phosphorites, and large deposits exist in Florida and the Western United States. These deposits generally contain from 50 to 200 ppm uranium (0.005 to 0.02%, or 0.1 to 0.4 pounds per ton). Although these concentrations are only 5% to 10% as high as those of commercially mined uranium ores, the vast extent of these deposits has made them of considerable interest as a uranium source for many years. It has been reported, for example, that mineable reserves of phosphate rock in the United States alone contain about 600,000 tons, or more than 1 billion pounds, of uranium.
A large and increasing portion of commercial phosphate production is converted first to a relatively dilute phosphoric acid by the so-called "wet-process" (as distinguished from the furnace process which produces elemental phosphorus by direct reduction of the ore). The producer first manufactures sulfuric acid, then uses it to digest the rock. The chemical reaction forms phosphoric acid and calcium sulfate. The latter is filtered out, providing enormous quantities of gypsum, a waste product, and leaving an acid stream typically containing about 30% P.sub.2 O.sub.5. Most of the uranium in the original rock shows up in the 30% acid, and various extraction processes have been developed to extract it therefrom. The 30% acid is generally evaporated to about 54% "merchant acid", which is either sold or used to manufacture a variety of products, chiefly fertilizers. The higher the acid concentration, the harder it is to extract the uranium, so the 30% stage is where the uranium extraction must take place. If uranium is not extracted, it ends up as a minor impurity in the various end products.
A number of prior processes have been developed to recover the minor amounts of uranium contained in wet-process phosphoric acid. In many of these processes, any hexavalent uranium is first reduced to the tetravalent state by the addition of iron and then extracted by contacting the acid with an organic extractant which has a high extraction coefficient (E.sub.a.sup.o) for uranium in the tetravalent state. As is known, the coefficient of extraction (E.sub.a.sup.o) is a measure of the extraction power of a reagent and is defined as the ratio of the concentration of uranium in the organic phase to the concentration of uranium in the aqueous phase at equilibrium.
The United States Atomic Energy Commission has devoted considerable effort to the recovery of uranium from wet-process phosphoric acid beginning in the early 1950's. Primarily as a result of these efforts, the discovery was made that mixed organic phosphoric acid esters, such as pyrophosphoric acid esters of octyl alcohol, are good extractants for uranium in the tetravalent state. Continued research in this area led to the discovery by Murthy et al in 1970 (IAEA-SM-135/11) that a mixture of orthophosphoric acid esters of octylphenol has a higher extraction coefficient at corresponding emf of phosphoric acid than the monoesters, such as the octyl, isodecyl, and tridecyl esters of phosphoric acid. All these mixtures are more desirable than the pyrophosphoric acid esters because of their inherent stability; that is their slow rate of hydrolysis in comparison to the extremely high hydrolysis rates encountered when using the pyrophosphoric acid esters as first proposed in the early 1950's.
Murthy et al's work was continued by the Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee, which demonstrated the process in bench-scale mixer-settler tests as reported in 1974 [Hurst et al, Ind. Eng. Chem., Process Des. Develop., 13,286]. However, subsequent work at ORNL has shown a selective loss of one of the mixed esters on repeated recycling of the reagent against phosphoric acid [Report 1976, Conf-760203-1].
We have found that the loss in extraction capability also observed by ORNL results from the precipitation of a ferric salt of the mixed esters. The analysis of this yellow precipitate has been established quite consistently. The apparent explanation for this precipitation is that most reagents that will extract uranium from phosphoric acid also have a significant affinity for other ions present, especially ferric ions. It is for this reason that uranium extraction is increased as the ferric ion concentration is reduced by substantial reduction of the emf of the phosphoric acid which is a measure of the ratio of the Fe.sup.+3 /Fe.sup.+2. Since it is not economical or practical to undertake complete elimination of the ferric ion from wet-process phosphoric acid, it is desirable to find a means of eliminating the precipitation of the ferric salt of the mixed esters.
Accordingly, it is an object of the present invention to provide an improved process for extracting uranium from wet-process phosphoric acid.
A further object of the present invention is to provide an improved process for extracting uranium from wet-process phosphoric acid using mono- and di-(alkylphenyl) esters of orthophosphoric acid.
Still a further object of the present invention is to provide a process for extracting uranium from wet-process phosphoric acid using mono- and di-(alkylphenyl) esters of orthophosphoric acid in which the losses of the extractant are minimized.
Yet a further object of the present invention is to provide a process for extracting uranium from wet-process phosphoric acid having a lower P.sub.2 O.sub.5 concentration than is possible using currently developed processes.
A still further object of the present invention is to provide a process for extracting uranium from wet-process phosphoric acid which is economical and minimizes the consumption of costly reagents.